This invention relates to a Field Effect Transistor (FET) device which is used for a solid-state relay which has no contact and which is mechanically operable, and a solid-state relay thereof.
In general, an FET device of the type described includes a pair of input terminals and a pair of output terminals. In this condition, the FET device carries out an on-off operation in response to a control signal applied to the input terminals, and supplies an output signal into an integrated circuit (IC) which is connected to the output terminals as a load circuit. Thus, the FET device is used as a solid-state relay for an IC tester and the like.
In such a solid-state relay, the output signal which is supplied from the solid-state relay often has high frequency with high density of the IC. To this end, it is required that the solid-state relay itself has such a structure that a high frequency signal can pass therethrough.
Therefore, the FET as a solid switch device that constitutes a circuit structure and the solid-state relay has been generally improved in such a solid-state relay, which the high frequency signal must be passed as the output signal.
Conventionally, the solid-state relay, which is combined FETs with a thyristor, is disclosed in Japanese Unexamined Patent Publication No. Sho. 63-2422 (hereinafter referred to as a first reference). In this event, the first reference discloses the solid-state relay which achieves high speed by improving the circuit structure.
Further, an example of a MOSFET which is used for the solid-state relay is disclosed in Japanese Unexamined Patent Publication No. Sho. 62-12167 (hereinafter referred to as a second reference). The MOSFET is a vertical type, and operable with enhancement mode.
Moreover, another solid-state relay is disclosed in Japanese Unexamined Patent Publication No. Hei. 5-41653 (thereinafter, referred to as a third reference).
In the solid-state relay, controlling two input signals can control polarity of current, which flows through a load circuit. Further, an exclusive OR between two input signals can be obtained as an output signal of the solid-state relay.
In this case, a product (namely, CR product) between device capacitance and on-resistance is generally used is a factor for evaluating a device that is used as the solid-state relay. In this event, when the CR product is small, the output signal of high frequency can be controlled.
In this event, when the solid-state relay in the first reference is used to control the high frequency signal, the high frequency signal passes between a pair of output terminals. It is necessary to use a device having a small CR product in the solid-state relay for passing the high frequency signal. This is because the solid-state relay can pass the high frequency as the CR product between the output terminals is small.
On the other hand, when the MOSFET itself in the second reference is used, a device breakdown voltage (namely, device breakdown voltage between a drain and a source during an off state) is about 50 V, the on resistance is about 10.OMEGA., and the CR product is approximately 20 pF.multidot..OMEGA.. These values are extremely small as compared to the other devices.
Under such a circumstance, the MOSFET having the small CR product is generally used in the solid-state relay. In this event, when the solid-state relay in the first reference is structured by using the MOSFET in the second reference, it is confirmed that the CR product between a pair of output terminals is approximately 30 pF.multidot..OMEGA..
As mentioned before, it is recently required to control the high frequency signal by the use of the solid-state relay in the market regarding communication. However, the CR product of the conventional solid-state relay has a limit of 30 pF.multidot..OMEGA. (the device breakdown voltage is about 50 V, and the on-resistance is approximately 10.OMEGA.) under zero-bias. Only a signal, which has frequency less than several MHz, can be controlled by the above CR product. In other words, it is found out that it is difficult to use the above-mentioned solid-state relay so as to control the high frequency signal exceeding several MHz.
Moreover, it may be possible to reduce the CR product of the solid-state relay in the first reference by reducing the CR product of the MOSFET itself. However, it is also confirmed that it is technically difficult to further reduce the CR product by the use of the MOSFET itself.
On the other hand, the above-mentioned third reference discloses the solid-state relay for obtaining the output signal in accordance with the two input signals. Namely, although the third reference discloses the solid-state relay which obtains the output signal corresponding to the two input signals via the two output terminals, no attention is paid for such a case that the output signal which is directly irrespective of the input signals is given to the output terminal, and the output signal is turned on and off.
In addition, the third reference does not suggest measures for the case that the output signal, which is turned on and off, has the high frequency.